通過外鍵查找Max（）值

Question

考慮以下數據庫結構：

 __________                 __________
| Trucks   |               | Mileage  |
|__________|__________     |__________|________________________
| ID    | DRIVER      |    | TRUCK_ID  | MILEAGE  | OIL_CHANGE |
|---------------------|    |-----------------------------------|
| 1     | Tony        |    | 1         | 100000     105000     |
| 2     | George      |    | 2         | 6020       10020      |
| 3     | Mary        |    | 3         | 37798      41000      |
|_____________________|    | 3         | 41233      47200      |
                           | 3         | 49000                 |
                           |___________________________________|

我想得到一個結果集，其中包含每個駕駛員的最大里程和最大機油變化。

_________________________________
| 1  | Tony  |  100000 | 105000 |
| 2  | George|  6020   | 10020  |
| 3  | Mary  |  49000  | 47200  |
|_______________________________|

到目前為止，這是我嘗試過的：

SELECT t.*, MAX(m.mileage) AS mileage, MAX(m.oil_change) AS oil_change
FROM trucks t
LEFT JOIN mileage m ON t.id = m.truck_id
GROUP BY t.id

但這似乎不允許MAX函數正常工作。 它並不總是包含mileage的實際最大值

Answer 1

得到它了！ 您的里程列必須定義為字符類型，而不是數字類型！ 如果發生這種情況，則按字母順序而不是按值順序進行排序。

您應該將轉換mileage和oil_change列數字類型（我建議INT根據所提供的數據樣本）。

當您不轉換它們時，它將起作用：

SELECT t.*, MAX(cast(m.mileage as int)) AS mileage, 
            MAX(cast(m.oil_change as int)) AS oil_change
FROM trucks t
LEFT JOIN mileage m ON t.id = m.truck_id
GROUP BY t.id

Answer 2

以下查詢應該可以解決您的問題。

SELECT T.DRIVER,MIN(MILEAGE) AS MIN_MILEAGE,MIN(OIL_CHANGE) AS MIN_OIL_CHANGE
FROM TRUCKS T INNER JOIN MILEAGE M
ON T.ID = M.TRUCK_ID
GROUP BY T.DRIVER;


SELECT T.DRIVER,MAX(MILEAGE) AS MAX_MILEAGE,MAX(OIL_CHANGE) AS MAX_OIL_CHANGE
FROM TRUCKS T INNER JOIN MILEAGE M
ON T.ID = M.TRUCK_ID
GROUP BY T.DRIVER;

關於Venk